This invention relates to ion-leachable inorganic compounds in the form of glasses and suitable for use as components of cements.
Ion-leachable inorganic compounds such as the oxides of aluminum, zinc, magnesium and calcium have been intermixed with other components such as silica and formed into glasses which, when combined with such hydrogen donating compounds such as acids, will set up into a cementitious mass. The mechanism for the reaction has been described by Alan D. Wilson et al. (J. Dent. Res. 58(3), 1065-1071, March 1979) and may be represented by the generic equation. ##EQU1##
H.sub.2 A may represent phosphoric or polyacrylic acid and M may represent Al as well as Zn, Mg or Ca.
Cements utilizing this mechanism have generally taken the form of glass powders incorporating the ion-leachable inorganic. These are reacted with liquid acid solutions such as aqueous carboxylic acid solutions to form a salt hydrogel structure which sets up into a hard mass. Such cement forming compositions have been suggested for use in applications such as dental cements and for orthopedic casts and splints. For example, a fluoroaluminosilicate glass powder has been suggested for use as the ion-leachable component for a dental cement in British Pat. No. 1,316,129. More recently, a similar composition has been suggested for use in orthopedic surgery in U.S. Pat. No. 4,143,018. In using such compositions for orthopedic purposes, for example, certain criteria must be met. The composition, when rendered reactive, must be capable of providing sufficient "working time", i.e. sufficient time from the start of mixing the reactants to allow the doctor time to apply and mold the cast into shape before the material reaches a stage where it is no longer malleable. Generally such times should be at most about 4 minutes and preferably from 1 to 2 minutes.
At the end of the working time period, it is most desirable that the cast set to a rock-like state as quickly as possible. While most cements, even after attaining a rock-like appearance, do not reach their ultimate strength for long periods of time, the material should reach sufficient compressive strength to allow a patient to leave the doctor's office, i.e., sufficiently hard enough to preclude deformation under expected stresses. This period is referred to as the "setting time" and should be about 6 to about 15 minutes after the cast is applied.
Within the frame work of providing practical working and setting times, perhaps the most important criterion to the user of such cements is predictability. When dealing with a patient, the orthopedist must be able to rely on the manufacturer's directions for predicting how much time he has to form a cast and when the patient will be free to leave the office.
From the foregoing it becomes apparent that a composition must be provided wherein the rate at which the salt hydrogel reaction proceeds corresponds to the constraints of working and setting times and is highly predictable and reproducible. As with most reactions, the rate is generally a factor of temperature and the availability of rate-limiting components which in this case is the availability of the leached metal ions from the powdered glass component. In practice, temperature is not generally a controlled factor in that the practitioner is accustomed to using aqueous mixture of components at essentially room temperature.
It thus becomes apparent that the manufacturer must supply a composition in which the rate of leaching of metallic ions is the controlling factor in meeting the various criteria of a satisfactory product. It is in this connection that the use of ion-leachable glass powders have heretofore been found wanting. Powders produced have varied greatly with respect to their physical and chemical homogeneity which in turn has produced erratic and unacceptable variations in working and setting times. In particular, it has been discovered that conventional glass frit making procedures have produced glass powders in which the components are separated into phases of different compositions, in which the micro-structure of the glass has been uneven, i.e., regions of amorphous material surrounding ordered regions of high crystallization, and in general, high degree of variance in amorphous to crystalline structure. While an exact correlation between rate of ion-leachability and micro-structure of glass powders is not known, reported studies have shown such a relationship between structure and rate, such work being reported by T. I. Barry, et al. in J. Dent. Res. 58(3): 1072-1079, March 1979. It has been discovered from this and other work that the rate of ion-leaching is substantially decreased when the degree of crystallinity of a glass is increased.
Accordingly, there is a need for providing glass powders in which the degree of crystallinity can be reproducibly controlled and predicted.